After submarine went missing, the president promised a full investigation.

As part of the probe, federal police raided naval bases and other buildings last January, following the dismissal of the head of the navy by the government.

The San Juan – a German-built diesel-electric TR-1700 class submarine – was commissioned in the mid-1980s.

It was refitted between 2008 and 2014 which involved it being cut in half and having its engines and batteries replaced.

The navy said previously the captain reported on November 15 that water had entered the snorkel and caused one of the sub’s batteries to short-circuit but later said it had been contained.

Some hours later, an explosion was detected near the time and place where the San Juan was last heard from.

The navy said the blast could have been caused by a “concentration of hydrogen” triggered by the battery problem reported by the captain.

Experts have said that refits can be difficult because they involve integrating systems produced by different manufacturers, and any mistake during the cutting phase can put the safety of the ship and crew at risk.